The present invention relates to a load driving circuit, and particularly to a load driving circuit suitable for the constant-current driving of a load.
For example, video tape recorders each comprise such a load driving circuit for the constant current driving of a load, which circuit serves as a reel motor driving circuit for driving a take-up reel.
In such a reel motor driving circuit, the driving current of a reel motor comprising a dc motor is controlled by employing the characteristic that the driving current is proportional to a torque, which can be generated by the dc motor, so that the tension of a magnetic tape is controlled to a constant value regardless of the rotational speed of the reel.
It is generally necessary to select a relatively large value as the output current value of the load driving circuit applied to such a reel motor driving circuit so as to cause the driving circuit to generate a torque sufficient for practical use. It is also necessary to use a power source with a sufficiently high voltage for practical use because the rotational speed of the reel motor varies within a wide range.
The torque of the reel motor comprising a dc motor is proportional to the value of the driving current thereof, and the rotational speed of the reel motor is proportional to the voltage under a constant load torque. The reel motor must rotate the reel at a relatively low speed during a recording/reproducing mode, while it must rotate the reel at speed when the magnetic tape is moved at a high rewind or fast-forward speeds.
It is therefore thought that the power loss of the load driving circuit which forms the output stage of the reel motor driving circuit is inevitably increased.